Many endpoints in IdentityServer will be accessed via Ajax calls from JavaScript-based clients.
Given that IdentityServer will most likely be hosted on a different origin than these clients, this implies that Cross-Origin Resource Sharing (CORS) will need to be configured.

One approach to configuring CORS is to use the AllowedCorsOrigins collection on the client configuration.
Simply add the origin of the client to the collection and the default configuration in IdentityServer will consult these values to allow cross-origin calls from the origins.

Note

Be sure to use an origin (not a URL) when configuring CORS. For example: https://foo:123/ is a URL, whereas https://foo:123 is an origin.

This default CORS implementation will be in use if you are using either the “in-memory” or EF-based client configuration that we provide.
If you define your own IClientStore, then you will need to implement your own custom CORS policy service (see below).

IdentityServer allows the hosting application to implement the ICorsPolicyService to completely control the CORS policy.

The single method to implement is: Task<bool>IsOriginAllowedAsync(stringorigin).
Return true if the origin is allowed, false otherwise.

Once implemented, simply register the implementation in DI and IdentityServer will then use your custom implementation.

DefaultCorsPolicyService

If you simply wish to hard-code a set of allowed origins, then there is a pre-built ICorsPolicyService implementation you can use called DefaultCorsPolicyService.
This would be configured as a singleton in DI, and hard-coded with its AllowedOrigins collection, or setting the flag AllowAll to true to allow all origins.
For example, in ConfigureServices:

IdentityServer uses the CORS middleware from ASP.NET Core to provide its CORS implementation.
It is possible that your application that hosts IdentityServer might also require CORS for its own custom endpoints.
In general, both should work together in the same application.

Your code should use the documented CORS features from ASP.NET Core without regard to IdentityServer.
This means you should define policies and register the middleware as normal.
If your application defines policies in ConfigureServices, then those should continue to work in the same places you are using them (either where you configure the CORS middleware or where you use the MVC EnableCors attributes in your controller code).
If instead you define an inline policy in the use of the CORS middleware (via the policy builder callback), then that too should continue to work normally.

The one scenario where there might be a conflict between your use of the ASP.NET Core CORS services and IdentityServer is if you decide to create a custom ICorsPolicyProvider.
Given the design of the ASP.NET Core’s CORS services and middleware, IdentityServer implements its own custom ICorsPolicyProvider and registers it in the DI system.
Fortunately, the IdentityServer implementation is designed to use the decorator pattern to wrap any existing ICorsPolicyProvider that is already registered in DI.
What this means is that you can also implement the ICorsPolicyProvider, but it simply needs to be registered prior to IdentityServer in DI (e.g. in ConfigureServices).